1. Field of the Invention
The present invention relates generally to optical densitometers and more particularly, to an improved densitometer which can measure either a sample's reflectance factor or transmittance factor with equally high accuracy.
2. Description of the Prior Art
It is important, when evaluating the optical properties of a material, to reliably and accurately ascertain its reflectance factor and/or transmittance factor. The reflectance factor, R, is a measure of the amount of light which is reflected from a sample's surface. It is defined as the ratio of measured reflected flux from the specimen to the measured reflected flux from a perfect-reflecting, perfect diffusing material located in place of the sample. Standards for measuring reflectivity are set out in ANSI/ISO 5/4-1983, ANSI PH2.17-1985. The transmittance factor is a measure of how much light can pass through a sample. It is defined as the ratio of the measured flux transmitted by the specimen to the measured flux when the specimen is removed from the sampling aperture. Standards for measuring the transmittance factor are established in ANSI/ISO 5/2-1985, ANSI PH2.19-1986.
In practice, the reflectance factor is measured by shining light onto the sample's surface at an angle of approximately 45.degree. and measuring the intensity of light reflected perpendicularly to the surface (ANSI standards specify the exact geometry). To measure the transmittance factor light is shined perpendicularly onto one side of a sample and the intensity of light passing directly through it is measured (again, ANSI standards specify the exact geometry).
Although it is relatively simple to construct an instrument which precisely measures only one of these values, it is more difficult to build one capable of accurately measuring both the reflectance and transmittance factors. Higher intensity light is needed to measure the transmittance factor than the reflectance factor. However, it may be difficult to find a light source which performs optimally at both these high and low light levels. In practice some densitometers use a plurality of light sources, as does U.S. Pat. No. 3,102,202, issued to Sweet, although other combination detectors employ a single light source, such as that disclosed in U.S. Pat. No. 3,542,479, issued to Sibalis. Neither of these solutions is entirely satisfactory, however, because both designs have features which compromise measuring precision.
Applicant's invention avoids the limitations of both the single and multiple lamp densitometers and as a result it offers superior performance. Multiple lamp units such as Sweet's usually include complicated optical paths which guide the reflected or transmitted light to the detector. Depending on which lamp is illuminated either reflected or transmitted light will be directed to the detector. Although the use of two lamps allows both to be selected to provide optimal lighting for each technique, the complexity of the multiple optical paths necessary to transmit the light to the detector means these densitometers cannot be built to perform optimally for both techniques. In fact, these "combined" devices will be less accurate than separate "dedicated" densitometers able to measure only the reflectance factor or transmittance factor.
As already noted, the design of single lamp densitometers able to measure transmittance and reflectance optical properties involves similar compromises which cause measuring inaccuracies. A single lamp may not emit light having the proper frequency distribution for differing intensities, and this reduces measuring accuracy. Single lamp densitometers also usually have several light detectors. Although the light paths are simplified, the costs are increased by the additional circuitry. Instrument calibration also becomes more complicated. See, for example, U.S. Pat. No. 4,352,988 to Ishida.